Bullet trace (what gives?) visible then not...

Dwayne Cyr said:
The O.P. lists 14 twist on the gun that produces visible trace. Plug 180 grain bullet and 14 twist into the Berger twist calculator http://www.bergerbullets.com/twist-rate-calculator/ There is a note that says "the BC of the bullet may not be optimized." I think that's because it will yaw all the way to the target and disrupt the air making visible trace. The minimum twist recommended for a 180 grain bullet is 12.8". Berger lists an alternate 115 grain bullet for greater stability in 14 twist.

In my humble opinion I think the slow twist is not delivering the bullet point forward and its yawing causing air disruption and visible trace.

D.C.
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Well I think you have it a bit backwards. He wants to see the swirl from the 30-06. Tumbling bullets make even bigger swirl than ones going straight. So he sees the swirl from the .308 but not the .30-06.
 
Go try again. Set up a spotting scope right behind your buddy with the 30-06. Make sure the sun isn't in your eyes.
Relax your eye and watch. You'll see the swirl I promise.
 
One possibility that nobody has touched on is the Optics. Not saying the optics are #### on the 308 ( which would be used to spot the 30-06 ) but the scope on the 308 may not be focused ideally for spotting swirl. A old trick for picking up trace is to adjust the optic just a hair out of focus. Normally a optic is focused at the target, but you are picking up swirl before it gets that far.
 
Low pressure is what causes the visible swirl. Like the vortices off the edge a heavy aircraft's flaps while landing in humid weather, the bullet's swirl is the same.

As it pushes through the air, the bullet is displacing air. Surrounding the bullet is higher pressure air, since it is being compressed and heated to get out of the way. As the air returns to its original position behind the bullet, it rapidly returns to normal pressure, and cools again. This causes the moisture in the displaced air to be expelled, and to become visible.

Perhaps the difference between the two bullets stated is bullet shape (a boattail will return the air to normal pressure more gradually) or speed. Speed may cause so much air displacement that the moisture dissipates elsewhere, like to the sides of the bullet. The moisture can't return to its position behind the bullet, as it's molecules are larger than air, and water is deflected away into warmer air further around the bullet's path.
 
Longshot said:
Well I think you have it a bit backwards. He wants to see the swirl from the 30-06. Tumbling bullets make even bigger swirl than ones going straight. So he sees the swirl from the .308 but not the .30-06.
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I think you got it backwards brother, 308 has 14 twist, op was shooting 180 grain bullets, possibly yawing slightly more compared to 30-06 which was shooting 165s. Not saying that's the cause of visible swirl or not just clarifying Dwayne's statement.
 
Thank You calgarycanada

Not wanting to get into it and waste energy quibbling about it. To my mind its like watching a well thrown football, flying straight and true. Or a poorly thrown football with the point yawing around in circles. Suffer through this video https://www.youtube.com/watch?feature=player_detailpage&v=SKbbuZDPw80 which has some good explanations. This shows what the point of the bullet is doing in flight. It also shows the bullet points up slightly as its falling. Its kinda like watching trace off the space shuttle during re entry.
 
If possible fire the same bullets out of each rifle. Same type and weight.

One other possibility. Could different rifling have an effect? Maybe the .308 has deep, sharp rifling and it's biting into the bullet a little more, where the .30-06 has smoother rifling, like 5r for example, and the "smoother" bullet isn't disrupting the air as much?
 
swissinn said:
Go try again. Set up a spotting scope right behind your buddy with the 30-06. Make sure the sun isn't in your eyes.
Relax your eye and watch. You'll see the swirl I promise.
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^^^^

Its there! Just get a spotting scope or even binos and set up like swissinn mention and you will see it. You or your buddy might not want to have you set up like that with a rifle looking through the scope. Doesn't even have to be a high-end spotter/bino either, I have a cheap bushnell 18-36x(less then 100bucks) that does a good job of it, I normally leave it set to 18x until we are on tgt.
 
Well gear has one thing, seems both pers can't see it through the 308's scope. I am guessing different scopes. They both could be the best but there maybe something different. 9x with one, may not equal 9x with the other kind of thing. That is why I say try spotting scope or binos, takes one variable out.

Next you have two different rounds shooting at different speeds with different projectiles. The projectile will be at different spots, and maybe looking in the wrong spot.

Example;

30-06

100yards 0.0inch
150yards -1.1inch
200yards -3.7inch
250yards -7.8inch
300yards -13.7inch
350yards -21.5inch
400yards -31.5inch
450yards -44.0inch
500yards -59.2inch

Range of Max Height 158yards

308

100yards 0.0inch
150yards -1.7inch
200yards -5.2inch
250yards -10.7inch
300yards -18.3inch
350yards -28.1inch
400yards -40.5inch
450yards -55.6inch
500yards -73.7inch

Range of Max Height 140yards

Take that info with a grain of salt, that is me just picking two of the same projectiles that equal the weights given and imputing the speeds, barrel length and twist rate you have indicated as that is all the info given.

The height difference might not seem like a lot but when looking through a rifle scope with a narrower field of view then a spotting scope it could be very easy to look in the wrong place.
 
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guntech said:
I don't believe that every round fired has a visible 'swirl'. A friend and I and the shooter first noticed this 'swirl' for the first time 47 years ago and that was before we had ever heard of it... I don't even remember the rifle/cartridge at that time... and we didn't see the swirl every time fired... but we saw it enough that we puzzled over it.

We concluded there was possibly some combustion gases trapped in the turbulence immediately behind the bullet that resulted in a 'puff' we could see for a millisecond when everything was right... is there any truth to this? What does cause this 'visible' swirl.
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Swirl is caused by the disturbance of the bullet travelling through the air. This causes the light traveling from the target to your eye to be bent slightly as it passes through the disturbed air over a long distance. It is the bent or deflected light that we see as swirl and not something physical coming off or following the bullet.

FWIW there is another kind of trace that the OP hinted at but nobody else has mentioned. It is entirely possible for a bullet to create a vapour trail, just like an aircraft. This requires very specific atmospheric conditions and I think is extremely rare. I have only seen it once but it was a very cool thing to see.

We were shooting 45-70 and larger falling block buffalo type rifles at 1000+ yds in central BC. Though the spotting scope, the vapour trail or trace looked like a large arrow like cone descending towards the target. It was so conspicuous one could almost call the shot prior to impact with the target.
 
Lol, I thought this answer pretty much covered it?

propliner said:
Low pressure is what causes the visible swirl. Like the vortices off the edge a heavy aircraft's flaps while landing in humid weather, the bullet's swirl is the same.

As it pushes through the air, the bullet is displacing air. Surrounding the bullet is higher pressure air, since it is being compressed and heated to get out of the way. As the air returns to its original position behind the bullet, it rapidly returns to normal pressure, and cools again. This causes the moisture in the displaced air to be expelled, and to become visible.

Perhaps the difference between the two bullets stated is bullet shape (a boattail will return the air to normal pressure more gradually) or speed. Speed may cause so much air displacement that the moisture dissipates elsewhere, like to the sides of the bullet. The moisture can't return to its position behind the bullet, as it's molecules are larger than air, and water is deflected away into warmer air further around the bullet's path.
Click to expand...
 
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